Line a large cookie sheet (13x18-inch) with parchment paper and spray with nonstick cooking spray. This stuff is sticky and we want to be extra safe in making sure it will release from the pan.
In a medium, heavy bottomed saucepan, stir together 2 cups granulated sugar, 1 cup light corn syrup, 1/2 cup water and 1/4 teaspoon salt then bring to a boil over medium heat. Hook the candy thermometer to the inside of the saucepan (making sure it is not touching the bottom of the pan or it won't read an accurate temperature.)
When the sugar comes to a boil (about 6 to 7 minutes) add 1 cup (2 sticks) unsalted butter, sliced. Stir to combine.
Let the mixture come back up to a boil until it reaches 280°F. Then stir in 2 1/4 cups unsalted, dry roasted peanuts.
Continue boiling the mixture until the candy thermometer reads 300°F (this is considered the hard crack stage.)
Remove from heat and stir in 1 teaspoon baking soda.
Immediately pour the mixture into the prepared cookie sheet and smooth it out.
Let cool completely. Break candy into pieces. Store in an airtight container.
Video
Notes
BEFORE MAKING: Please refer to my FAQ’s (Frequently Asked Questions) for the answers to the most common questions and to ensure this turns out properly for you.
You must use a candy thermometer and work in a low humidity environment for the brittle to set up. I can't stress this enough. Don't try to do this without a candy thermometer unless you are already a master at making brittle and know the exact consistency to look for. If the brittle does not reach a hard crack stage (300F degrees) it will not set up and harden properly.
Also, the correct pan size matters. If your pan is too small then the brittle mixture will be too thick and won't be able to harden up properly.
If you like it to have a more salty flavor, you can use salted peanuts.